Staphylococcus aureus possesses three MsrA enzymes (MsrA1, MsrA2, MsrA3) that reduce the S-epimer of methionine sulfoxide\n(MetO) and an MsrB enzyme that reduces R-MetO. The four msr genes are expressed from three different promoters. The\nmsrA1/msrB genes are coexpressed. To determine the expression pattern of msr genes, three independent reporter strains were\nconstructed where msr promoter was cloned in front of a promoterless lacZ and the resulting construct was integrated in the\nchromosome. Using these strains, it was determined that the msrA1/B expression is significantly higher in S. aureus compared to\nmsrA2 or msrA3. Expression of msrA1/B was highest during stationary phase growth, but the expression of msrA2 and msrA3 was\nhighest during the early to midexponential growth phase. Expression of msrA1/B was induced by oxacillin and the expression of\nmsrA3 was upregulated by salt. Expression of msrA2 remained unchanged under all tested conditions.
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